1. Field of the Invention
Single-stage, single-unit, rotary drum-granulator devices and their use in and for a variety of production operations are not new per se. They are used commercially, for example, in the production of granular phosphate and complete N-P-K fertilizers. In these devices, feedstocks such as acidified phosphate rock slurry, or phosphoric acid, ammonia and/or steam or water are injected onto or into a tumbling rolling bed of particulate material maintained in the granulator to effect agglomeration of the feed materials within such drum, with said bed of materials generally comprising fertilizer dust, fines, and/or onsize product granules. It has long been known that efficiently and effectively causing the granulation or agglomeration of such particulate material in such drum requires a relatively high degree of moisture for granule formation and that a significant portion of such moisture, which is required for effecting such particle formation, must subsequently be removed so as to enhance the mechanical stability, storage, and handling characteristics of the resulting agglomerate formed therein. For instance, in the commercial production of superphosphate fertilizer, the high-moisture, triple superphosphate (TSP) slurry is introduced into the granulator device which contains a tumbling, rolling, free flowing bed of materials comprising dust, fines, crushed oversize, undersize, and onsize product and therein agglomeration of the resulting maintained and newly introduced dry solids materials and high-moisture slurry occurs. However, in order to attain the desired, necessary, and satisfactory mechanical stability of the resulting formed particles, it is essential that a large portion of the water, originally introduced as a significant constituent of such slurry, be removed.
In the commercial practice of granulation processes such as, for example, the production of granular TSP, in which there is little or no in situ heat generated in the granulator by the combining of feedstock materials therein, it is often beneficial to supply supplemental heat to the granulator unit directly as stem or indirectly by preheating the feedstock materials fed thereto. It will also be appreciated that materials which are granulated in such units usually are discharged therefrom and thereafter introduced into another stand alone unit, such as a rotary dryer, wherein heat energy is added to effect additional drying of the resulting granular product.
2. Description of the Prior Art
It is generally conceded that the most commonly used device for granulating fertilizer is the rotary-drum ammoniator-granulator as described in U.S. Pat. No. 2,741,545, Nielsson, Apr. 10, 1956, assigned to the assignee of the present invention. This single-unit apparatus is usually equipped with an ammonia sparger and a solution sparger. The ammonia sparger is generally located so as to be down in and near the center of the granulation bed during actual operation of such apparatus. Likewise, the solution sparger may also be located down in the granulation bed or it may be located above the granulation bed. Feedstock to the granulator unit may consist only of TSP slurry as in the production of granular TSP; as acid solution or as partially ammoniated acid solution and ammonia as in production of monoammonium or diammonium phosphates; or as solid material such as ammonium sulfate crystals, acid solution, and ammonia as described in the process disclosed in U.S. Pat. No. 4,589,904, Harrison, et al., May 20, 1986, assigned to the assignee of the present invention, for granulating by-product ammonium sulfate crystals. The solution feed as acid, partially ammoniated acid, ammonia solution, or slurry to the granulation unit usually contains a relatively high content of water which is necessary to promote granulation of the dry solids materials; however, the major portion of the introduced water and water that may be formed by chemical reaction in the granulator must be removed from the agglomerates or particles formed in order to obtain mechanical stability and to assure satisfactory handling and storage properties of the resulting granules. The feedstock entering the granulator usually contains at least about 3 percent and oftentimes 10 percent or more by weight free moisture. It is therefore necessary to provide for the additional drying of most of such granular products, since without such additional drying, most of same have been found to not be completely free flowing and to exhibit undesirable tendencies to cake during even short periods of storage.
Currently, large tonnages of granular TSP are produced by first acidulating phosphate rock with phosphoric acid and thereafter granulating the resulting superphosphate slurry in a granulation device of a basic design similar to that described in '545, supra. In such processing operations, fine ground phosphate rock is acidulated with phosphoric acid of about 37 percent to 42 percent P.sub.2 O.sub.5 concentration. In the acidulation step, the proportions of acid to rock are adjusted or predetermined to provide for a weight ratio of acid P.sub.2 O.sub.5 to P.sub.2 O.sub.5 of about 2.3:1 to about 2.5:1. After about 90 minutes of acidulation, the resulting slurry, at a temperature of about 190.degree. F. to 200.degree. F. and containing about 15 percent to 20 percent water, is sprayed onto a bed of recycle and product-size granules maintained in the granulator. While in the ensuing free flowing condition, i.e., rolling and tumbling, in the rotating granulator device, moisture transfer from the slurry to the dry recycle material together with moisture loss by evaporation helps to cause the introduced slurry and dry recycle material to form granules. The granules formed usually contain about 3 percent by weight of moisture as discharged from the granulator device; consequently, additional drying is required to obtain a product with satisfactory handling and storage properties. Accordingly, the product discharged from the lower end of the granulator is subsequently introduced into a separate single-unit dryer which is equipped with a combustion chamber located at the solids feed end of such unit. As the granular product passes through the dryer unit, hot gases are caused to flow cocurrent with respect to the flow of such granular material through the dryer unit to thereby cause the moisture content of such granular product, when subsequently cooled, to be reduced to an acceptable level. The hot gases, still containing a significant quantity of drying potential energy, flow from the dryer unit to the gas-scrubbing system for venting, wherefrom such valuable potential energy values still present or contained therein are lost.
In still another granulating type device as, for example, that described in U.S. Pat. No. 4,650,682, Shirley, Mar. 17, 1987, therein is utilized a rotary drum containing both a first flighted compartment and a second nonflighted compartment which combination of compartments is designed for sequentially chemically treating particulate solid materials, in free-flowing condition, with fluid reactants, i.e., first with acid and subsequently with ammonia. A primary object of such treatment is to therein convert a product, such as poultry litter, into a feed supplement deemed suitable for ruminants.
It should be readily apparent to those skilled in this art that both the sequence of flow of materials through the two compartments of Shirley, '682 supra, as well as their physical arrangement is substantially different from that of the instant invention. In the instant invention, the first compartment of the two-compartment, single-unit, and energy-efficient granulator dryer is nonflighted and is the compartment wherein granulation of the input feed materials occurs. The second compartment of the instant two-compartment, single-unit, and energy-efficient granulator dryer is flighted and is where drying occurs. Granulation or agglomeration of feedstock fed to the granulation compartment is promoted by transfer of moisture from the feed materials or water formed by chemical reaction to the hot gases first entering and then exiting the dryer compartment of the instant two-stage, single-unit, and energy-efficient granulator dryer and then passing through the granulator compartment of the instant, new, novel, two-stage unit. It should also be appreciated by those skilled in this art that in the operation of the two-stage or two-component single-unit combination granulator-dryer device of the present invention, all chemical reactions occur in the granulator compartment and, for all practical purposes, no sequential chemical reactions occur thereafter, albeit, at least some of those reactions initiated in the first-stage or compartment may continue to completion therein. Accordingly, the first compartment-acidifier and second compartment-ammoniator described in '682, supra, simply would not be suitable for either granulating or for agglomerating and drying of fertilizer or nonfertilizer feedstock without the attendant utilization of another separate or combined dryer unit because, without such additional and separate drying equipment, there are no provisions for the necessary removal of water from the resulting product.
There are, of course, many other types of granulation equipment that are well known in and utilized by the chemical processing industry such as, for example, the pug mill, the blunger, and the pan granulator. It is also well known and appreciated that these devices are of the single-unit operation type and in the operation of most processes in which such equipment is utilized, other than certain melt granulation processes, a separate dryer unit is required in combination therewith, for final water removal from the resulting granular product.